The present invention relates to novel solid polymeric compositions, their preparation and cross-linking, and their uses.
A well-established method for improving the physical properties and solvent resistance of linear polymers is cross-linking, in which the individual polymer chains are joined at many points to yield an interconnected network. The cross-links may be ionic in character, as in complexes of poly-acids with poly-bases, but covalent or chemical cross-linking is stronger, more resistant to hydrolysis, and more versatile in its applications.
The most common way of achieving covalent cross-linking is to use a polyfunctional monomer in the polymerization reaction itself (e.g. divinylbenzene together with styrene). Since cross-linked polymers cannot be dissolved, melted or cast, the polymerization must be carried out in the final physical shape required. Problems often arise in the preparation of films and membranes when catalysts must be introduced or inhibitors such as oxygen excluded from the polymerization reaction. Furthermore, the monomers themselves are often too toxic, volatile, or fluid to be conveniently processed in this manner. All of these factors make the manufacturing process difficult and costly.
It is thus often preferable to prepare a linear polymer or polymers, dissolve them in an appropriate solvent, spray or cast in the final form required, and only then introduce the cross-links. A familiar example of such post-cross-linking is the vulcanization of rubber, in which linear polyisoprene is mixed with sulfur, molded, and then heat-cured. Another example is the cross-linking of various unsaturated polymers by light to produce plates for photoengraving.
Although many polymers will form loosely cross-linked gels upon heating, the mechanism is often obscure and the cross-linking difficult to predict or control. The present invention provides a post-cross-linking method unique in its mechanism, in the range of polymers to which it applies, and in its practicality and wide range of application. It is believed to proceed via the alcoholysis of pendant amide or ester groups on the polymer to produce a new ester linkage between polymer chains. Alcoholysis of amides and esters is a well-known reaction in the organic chemistry of small molecules, where it generally is run in solution at reflux temperature with strong acid or base catalysis. It is one of several mechanisms postulated by Kopecek and Bazilova to account for unusually high molecular weights obtained in solution polymerization of N-(2-hydroxypropyl)methacrylamide (European Polymer Journal, 1973, vol. 9, pp. 10-11.) These authors considered only the possibility of dimerization, not actual cross-linking, and did not report any insolubilization taking place. The unusual feature of the alcoholysis reactions described in this invention is their occurrence in the solid state in the absence of solvent, a phenomenon entirely unexpected and heretofore unreported.
The novel post-cross-linking techniques taught by this invention have many unique advantages, particularly in the manufacture of synthetic membranes. Conventional membranes for reverse osmosis (RO) and ultrafiltration (UF) are made of polymers that are insoluble in the fluid acted on by the membrane (water, in most cases). Typically, a linear polyamide, polysulfone, or cellulose acetate is cast from an organic solvent and coagulated in water. Although such membranes are rigid and physically strong, they are hydrophobic in nature and tend to foul through adsorption of hydrophobic particles and solutes in the feed stream. Such fouling is a major problem in industrial use of membranes, making frequent cleaning or costly pretreatment necessary.
Although hydrophilic polymers, particularly highly sulfonated ones, have been shown to resist such adsorptive fouling, they either dissolve in water or form a soft gel. Useful membranes may be formed of such polymers only by a high degree of cross-linking, so they swell not more than 3-5 times by weight when soaked in water. Since the pore structure of such membranes is generally created by coagulation and since monomers are essentially uncoagulable, the only practical route is post-cross-linking of the coagulated linear polymer. Using the techniques taught by this invention, highly cross-linked UF membranes of controlled porosity may be cast from very hydrophilic polymers. The intricate pore structure of a coagulated membrane is preserved by solid state post-cross-linking. Furthermore, charged functionalities such as sulfonate or quaternary ammonium may be incorporated to yield a cross-linked interpolymer membrane of the type described by Gregor (U.S. Pat. No. 3,808,305), where the fixed charges serve to reject charged colloidal particles and, to some extent, dissolved salts.
The teachings of this invention are applicable to the preparation of virtually all known kinds of polymer membranes. For example, conventional RO membranes tend to suffer compaction or swelling over a period of time and are highly susceptible to biological and chemical attack (particularly chlorination). The tight cross-linking made possible by the teachings of this invention provides physical strength and mitigates the effect of polymer chain scission, thus enabling RO membranes to keep their selectivity under conditions of use. Furthermore, one may prepare an RO membrane with a hydrophilic, non-fouling coating, thus reducing the need for pretreatment of the feed stream.
Conventional electrodialysis (ED) membranes contain a small percentage of charged groups fixed inside the pores of a hydrophobic polymer matrix. Using the teachings of this invention, ED membranes with very high fixed charged densities, tight pore structures and low swelling may be prepared at low cost. Membrane permeation (MP) or pervaporation requires an extremely thin film which selectively adsorbs one component of the feed stream without irreversible swelling or shrinking. The teachings of this invention allow one to incorporate any of a wide range of polymers with different chemical affinities into a film whose swelling is determined by the degree of cross-linking. Solvent extraction membranes must similarly resist excessive swelling but with controlled porosity, and these techniques are again applicable. Finally, the wide range of functional groups compatible with this method of post-cross-linking allows for unusual versatility in the preparation of porous, coagulated membranes for enzyme coupling.
The teachings of this invention enable one to make a wide range of novel and highly useful adhesive coatings. Cross-linked coatings have long been known to exhibit superior strength and solvent resistance, but their preparation generally involves either photolytic curing (as with paints) or noxious chemical monomers (as with epoxies). The present invention allows a viscous solution of a polymer and relatively innocuous cross-linking agents (which may be incorporated in the polymer) to be applied to a surface and cured by heat in a relatively short time. Polyacrylamide and its homologs may be cast into thin films suitable for electrophoresis. Coatings containing highly sulfonated polymers are anti-fouling, non-thrombogenic, and resist the adhesion of micro-organisms. They are thus useful for such purposes as coating heat exchangers, artificial heart valves, ship bottoms, and screens for suspended solids removal. It is also possible to make solid gels according to the teachings of this invention which combine high water content, high physical strength, biocompatibility, and resistance to biocontamination with optical clarity, making them ideal for long-wearing contact lenses and other devices used in eye care.